Brutus is an introvert; Cassius is an extravert. Brutus is often shown alone. Cassius is always with someone in the play.

Brutus is an idealist; Cassius is a realist.

Brutus is unselfish; Cassius is a greedy miser.

Brutus is self-sufficient; Cassius is competitive.

Brutus is honest; Cassius is typically dishonest.

Brutus is bookish; Cassius is worldly wise.

Brutus is likeable; Cassius is not liked by most people, including Caesar.

It should be remembered that Shakespeare's characters are not the real Brutus and Cassius of history. I think Shakespeare got most of his information from Plutarch, in translation. Shakespeare wanted to give his characters striking distinguishing character traits in order to differentiate them for his audience. It is hard to understand why Shakespeare's Brutus and Cassius should be such good friends, since they are so different. Maybe Cassius is trying to disguise his true nature most of the time. He has different reasons than Brutus for wanting to assassinate Caesar. He wants power and wealth. He is also afraid of Caesar, who might actually have Cassius killed or exiled if he became absolute ruler.

I think it is interesting to look at the differences between Brutus and Cassius from Caesar's perspective. According to Caesar, Cassius has a lean, hungry look:

Cassius over there has a lean and hungry look;He thinks too much. Such men are dangerous.

Caesar is very perceptive. He realizes Cassius is dangerous.

No doubt, Cassius is very critical of Caesar when he and Brutus are talking. Cassius is greedy himself. He uses deceit and false petitions to convince Brutus to join the conspirators.

Brutus is an honorable man. He loves Caesar, but he loves Rome more:

Not that I loved Caesar less, but that I loved Romemore. Would you rather Caesar were living, and you all die slaves, than that Caesar were dead, so you all live freemen? As Caesar loved me, weep for him; as he was fortunate, I rejoice at it; as he was valiant, I honor him; but, as he was ambitious, I killed him.There are tears for his love; joy for his fortune; honor for his valor, and death for his ambition. Who is here so low that they would rather be a slave? If there are any, speak, because I have offended him.

Brutus had good intentions when he slew Caesar. He admits that loved Caesar and that Caesar loved him. More than anything else, Caesar's dying words prove that Brutus is an honorable man. When Caesar realizes that Brutus is in on the conspiracy, he cries out "Et tu Brute? Then fall Caesar."

Caesar is in agreement with Brutus that if Brutus sees the need to kill Caesar, then Caesar must need to die. That is the confidence that Caesar had in Brutus.

As Brutus falls upon his own sword in death, Antony proclaims that Brutus was an honorable man:

This was the noblest Roman of them all.All the conspirators, except him,Did that they did out of jealousy of great Caesar;Only he, in a general-honest thoughtAnd common good to all, made one of them.His life was gentle, and the elementsSo mixed in him that Nature might stand upAnd say to all the world, "This was a man!"